单例理论 - PHP

class dbcontroller{    private static $instance;    public static $db;    private function __construct()    {        $this->db = new pdo("mysql:host=localhost;dbname=***;", "root", "");    }    public static function getinstance()    {        if (!isset(self::$instance)) {            self::$instance = new dbcontroller;        }        return self::$instance;    }    public function dbconnection()    {        if (!isset(self::$db)) {            self::$db = new pdo("mysql:host=localhost;dbname=***;", "root", "");        }        return self::$db;    }}

$cont1 = DbController::getInstance();$cont2 = DbController::getInstance();var_dump($cont1);var_dump($cont2);if ($cont1 === $cont2) echo 'Same';